Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore, other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersions used in coating processes are usually in a concentrated form and contain a significant quantity of nonionic surfactant, e.g. 6-8 weight percent, as taught in Marks et al., U.S. Pat. No. 3,037,953, and in Holmes, U.S. Pat. No. 3,704,272 to raise the solids from nominally 35 wt % in the raw dispersion to about 60 wt % in the concentrated dispersion. Miura et al., U.S. Pat. No. 6,153,688 discloses a similar process. Similar dispersions and coating processes are also used for making coatings of melt-processible fluoropolymers.
Fluorosurfactants are typically used as an ingredient in the dispersion polymerization of fluoropolymers since a non-telogenic dispersing agent is generally required in commercial fluoropolymer dispersion polymerization processes. For example, an early description of fluorosurfactants used commercially is found in U.S. Pat. No. 2,559,752 to Berry. These fluorosurfactants are anionic surfactants, usually perfluorinated carboxylic acids, e.g., ammonium perfluorocaprylate or ammonium perfluorooctanoate.
Because of environmental concerns and because fluorosurfactants are expensive, processes have been developed for the removal and recovery of fluorosurfactants from aqueous fluoropolymer dispersions. One method for removal of fluorosurfactants from fluoropolymer dispersions is disclosed in U.S. Pat. No. 4,369,266 and includes the addition of a stabilizing surfactant followed by concentration by ultrafiltration. This patent teaches that a high proportion of the fluorosurfactant can be removed via the aqueous permeate. It is also known to remove anionic fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls). Kuhls teaches recovery of fluorosurfactants dissolved in the aqueous phase after coagulation of the polymer from the dispersion or in aqueous polymer dispersions to be concentrated. U.S. Pat. No. 6,833,403 (Bladel et al.) is a more recent patent teaching the same process. In these anion exchange processes, the fluorosurfactant is removed by the anion exchange resin from a stabilized dispersion containing nonionic surfactant.
In concentrated aqueous fluoropolymer dispersions which have reduced levels of fluorosurfactant, the viscosity levels can be higher than in dispersions containing fluorosurfactant and can be unacceptably high for some end uses. Certain types of fluoropolymer dispersion, particularly high molecular weight polytetrafluoroethylene dispersions, show an increase to an unusually high viscosity when the fluorosurfactant content is significantly reduced. Viscosity can rise to a level of several hundred centipoise (cP), well above the normal 20-30 cP which is advantageous for coating and impregnating compositions and to make cast films. US 2004/0186219 A1 and U.S. Pat. No. 6,861,466 B2 (Dadelas et al.) disclose the addition of non-fluorinated anionic surfactant, e.g., sodium lauryl sulfate, sodium dodecylbenzyl sulphonate and secondary alkyl sulphonate sodium salt, to reduce viscosity. These references teach the addition of non-fluorinated anionic surfactant prior to or after concentration.
However, employing non-fluorinated anionic surfactant according to the processes of US 2004/0186219 A1 and U.S. Pat. No. 6,861,466 B2 (Dadelas et al.) can cause problems when concentration is carried out using a thermal concentration method as taught as taught in Marks et al., U.S. Pat. No. 3,037,953, and the other patents referred to above. In thermal concentration, the lower solubility of the nonionic surfactant at elevated temperatures, i.e., the nonionic surfactant having “cloud point”, is used to create a phase separation with a lower phase having high fluoropolymer solids and an upper phase having very low solids. These phase are separated usually by decanting off the upper phase and recovering the lower high fluoropolymer solids phase as concentrated dispersion.
When non-fluorinated anionic surfactant is present during concentration, a significant portion of it will be lost due to it being in the low solids upper phase after concentration. If sufficient anionic surfactant is added prior to concentration to compensate for such loss and to control viscosity in the finished concentrated dispersion, the anionic surfactant has been found to narrow the “concentration window”. The “concentration window” refers collectively to the range in the level of nonionic surfactant and the temperature range within which concentration will proceed efficiently. For concentration with a narrow concentration window caused by too much anionic surfactant, tight control of and/or more extreme levels of nonionic surfactant, temperatures or both will be required. Moreover, the amount of non-fluorinated anionic surfactant will vary greatly with the exact solids content of the finished dispersion, i.e., slightly higher that the targeted solids content will reduce anionic surfactant levels. This will cause the non-fluorinated anionic surfactant level to vary from batch to batch and the level in some dispersions may be insufficient to control viscosity.
If, on the other hand, the addition of the anionic surfactant is delayed until after concentration, the concentrated dispersion without anionic surfactant will go through an extremely high viscosity maximum which may interfere with handling of the dispersion for decanting of the upper phase and in subsequent handling prior to anionic surfactant addition.
In addition, a phenomena known as “skinning” can occur in fluoropolymer dispersions. When a dispersion is allowed to stand in an open, stagnant bath, a skin can sometimes form on the surface of the dispersion due to drying. This skin is detrimental in that it can sometimes be picked up by the article being coated and result in a coating defect. While skinning can occur in several types of dispersions, it is more pronounced in dispersions having a high viscosity and/or a low level of the fluorocarbon surfactant used in polymerization.